Filters for filtering gases provided in a clean room for use in the manufacture of integrated circuits or in a cooled air circulating duct in a driving apparatus for floppy discs are required to have such performance that they catch a fine dust that is elusive by the use of commercially available common filters. Concretely speaking, filters for filtering gases under present conditions are required to catch a dust having a diameter of larger than 0.3 .mu.m, and with the progress of high densification of integrated circuits, there is a strong and growing demand for high-performance filters capable of catching a dust finer than that mentioned above.
As fully demonstrated, for example, in Japanese Patent L-O-P Publn. No. 225416/1985, electrically charged synthetic fiber nonwoven fabrics have widely been used as filters for catching such finer dust as mentioned above.
Requirements for electret filters are such that the filters maintain stably and continuously their state of being highly densified with electrical charge over an extended period of time and, at the same time, they are free from a phenomenon of making dust by themselves, that is, self-dusting properties.
Under the circumstances, electrically charged synthetic fiber nonwoven fabrics have been subjected to needle punch treatment in order to prevent the nonwoven fabrics constituting electret filters from falling into pieces. When the electrically charged synthetic fiber fabrics are subjected to needle punch treatment, however, parts of the fibers of the nonwoven fabrics are cut in some cases, resulting in a serious problem that the cut fibers from the determining cause of the self-dusting properties of the electret filters as aforesaid.
In the needle punch treated electret filters, the surface thereof is fluffy, and this is considered to be one of the causes of the self-dusting properties. Further, the needle punch treated filters had such a problem that difficulty is involved when the filters are subjected to pleating (gathering), because they cannot be made thin and are poor in toughness.
On the one hand, it is thought of as possible to integrate electrically charged synthetic fiber nonwoven fabrics into a solid structure by means of ultrasonic bonding, but it has been found that difficulty is involved in pleating the ultrasonic bonded electret filters, because the filters cannot be made thin and are poor in toughness.
In order to solve such problems as mentioned above, it is thought of as possible to integrate the nonwoven fabrics constituting the electret filter into a solid structure by subjecting the electrically charged synthetic fiber nonwoven fabrics to heat bonding, but it was considered that when the electrically charged synthetic fiber nonwoven fabrics are heat bonded, the electrical charge once imparted to the synthetic fiber nonwoven fabrics would come to markedly decrease at the time of heating.
Accordingly, there has been earnestly desired the advent of such a process wherein electrically charged synthetic fiber nonwoven fabrics can be integrated into a strong solid structure and used as filters without largely decreasing the electrical charge imparted to said nonwoven fabrics.
Electret filters comprising the electrically charged synthetic fiber nonwoven fabrics integrated into a solid structure are used as filters for air conditioner, because the filters have a fairly acceptable strength for their small thickness and a good performance of catching dust particles.
However, such electret filters sometimes break or spread at the time of fabrication process thereof, because the filters are insufficient in mechanical strength.
Because of lack of self-supporting properties, the above-mentioned electret filters were sometimes found to be difficult to handle particularly when they are stacked.
With the view of obviating such difficulities as mentioned above, there have heretofore been proposed and known reinforced type electret filters prepared by ultrasonic bonding the four sides of the filter to a net made of polyethylene terephthalate as a packing material. However, this ultrasonic bonding is difficult to perform and, moreover, there were such problems that the reinforced type electret filters are low in self-supporting properties and difficult to handle, because only four sides of the electret filters are bonded to the polyethylene terephthalate net as a packing material while the greater part of the nonwoven fabrics constituting the filter are not bonded to said net.
In light of such circumstances as mentioned hereinbefore, there has been strongly desired an advent of electret filters which are of easy handling by virtue of improved mechanical strength and self-supporting properties, and of a process for preparing the same.